This invention relates to electrodes for high-temperature, secondary electrochemical cells that can be formed into batteries and used as power sources for electric vehicles, storage of off-peak generated electrical power and various other applications. It is particularly applicable to positive electrodes employing metal sulfides as active material within electrochemical cells with molten metal halide salt as electrolyte.
The type of electrochemical cells contemplated are those that include transition metal sulfides as the positive electrode reactant and alkali metals such as lithium in the form of solid alloys as the negative electrode material. As an alternative, alkaline earth metals such as calcium and alloys of these metals can be selected as negative electrode material. These high-temperature cells employ molten salt electrolytes generally including the halides of the alkali metals or of the alkaline earth metals. During operation of these cells the transition metal sulfide discharges to alkali metal sulfide or alkaline earth metal sulfide in the positive electrode. This electrochemical environment is very corrosive and has required particular attention to the selection of corrosive resistant materials of construction.
Various electrochemical cells with corrosive environments are illustrated in an number of earlier U.S. patents. U.S. Pat. No. 4,313,259 to Kaun et al illustrates an electrochemical cell with a plurality of electrodes with perforated electrically conductive metal containment as current collectors. Another cell design with plate-like electrodes is illustrated in U.S. Pat. No. 4,110,517 to Arntzen. A compartmented or honeycombed current collector structure is illustrated in U.S. Pat. No. 4,029,860, while U.S. Pat. No. 4,011,374 to Kaun shows the use of thermosetting resin as moldable material containing the electrode active material.
These high powered electrochemical cells with corrosive molten salt electrolyte at elevated temperatures have presented substantial problems in the selection of suitable corrosion resistant materials for containment structures and current collectors. Corrosion resistant materials such as molybdenum, nickel and chromium and alloys of these metals have been used in an effort to prolong cell life under such corrosive conditions. These materials are very expensive and add substantially to the cost of high powered electrochemical cells employing them.
In an effort to produce less expensive cells, FeS has been selected as the positive electrode active material in combination with current collectors of low-carbon steel. Corrosion rates with the FeS material are substantially less than those with the more reactive FeS.sub.2 active material in the positive electrode. However, after long term operation substantial corrosion occurs to low-carbon steel current collectors and containment structures in cells with FeS positive electrodes.
Therefore, in view of the above it is an object of the present invention to provide a positive electrode for use in a high-temperature, secondary electrochemical cell with improved corrosion characteristics respecting low-carbon steel components.
It is another object of the present invention to provide a positive electrode with iron sulfide active material in mixture with metal halide electrolytic salt with inexpensive low carbon steel components of extended life.
It is still another object to provide a positive electrode that exhibits low corrosion rates in its current collector components without use of costly corrosion resistant alloys.
It is a further object to provide an improved method of preparing a positive electrode for use in a high-temperature, secondary electrochemical cell with molten metal halide electrolyte to reduce corrosion of low-carbon steel components.
In accordance with the present invention, a positive electrode is presented for use in a high-temperature, secondary electrochemical cell opposite to a negative electrode with alkali metal or alkaline earth metal as active material. The positive electrode contains metal sulfide that can include FeS, alkali metal sulfide, alkaline earth metal sulfide or mixtures thereof depending on whether the electrode is in the charged, uncharged or partially charged state. The metal sulfide is in mixture with finely divided iron powder in sufficient amount to be a stoichiometric excess of Fe as FeS to the amount of combined sulfur in the metal sulfide. The mixture is in contact with a current collector structure consisting essentially a low-carbon steel.
In more specific aspects of the invention, the low carbon steel includes no more than 0.5% carbon and is substantially free of the alloying elements nickel, chromium and molybdenum commonly used to improve corrosion resistance. Such costly elements are included at no more than 0.1% by weight. In further aspects, the finely divided iron powder is included in uniform mixture with a metal halide electrolyte and particulate FeS. The finely divided powder of about 40 to 100 micrometers diameter is included at about 15-40 mole percent excess to that required to form FeS in the fully charged electrode.
In other aspects of the invention, a method is presented for preparing a positive electrode for use in a high-temperature, secondary electrochemical cell. The cell includes FeS as the positive electrode reactant, an alkali metal or alkaline earth metal as the negative reactant and a metal halide electrolyte. The formation of the positive electrode includes the step of adding finely divided iron powder in sufficient excess to the FeS to substantially improve the corrosion resistance of low-carbon steel current collector components contacting the positive electrode material.